I. Field
The present disclosure relates generally to electronic circuits, and more specifically to techniques and circuits that improve linearity in passive mixer circuits.
II. Background
A wireless communication device such as a cellular phone and/or near field communication (NFC) device typically includes a transmitter and a receiver to support bi-directional communication. The transmitter may upconvert output baseband signals with transmit local oscillator (LO) signals to obtain an output radio frequency (RF) signal that is more suitable for transmission via a wireless channel. The receiver may receive an input RF signal via the wireless channel and may downconvert the input RF signal with receive LO signals to obtain input baseband signals.
The receiver may include an inphase (I) mixer and a quadrature (Q) mixer to quadrature downconvert the received RF signal. An ideal mixer simply translates an input signal from one frequency to another frequency without distorting the input signal. Mixers can be active or passive.
RF transmissions by certain classes of wireless devices can entail transmission of a relatively large carrier signal. The signal received can in certain instances include the relatively large carrier signal and a relatively small desired signal or modulation signal. The receiver can have difficulty discerning this smaller desired modulation signal. The mixer swing can be large enough to degrade the linearity of the circuit and can cause a significant I/Q mismatch, resulting in a poorly resolved signal at the receiving device.
Previous mixers, such as passive mixers, have employed a series of switches that switch depending on signal levels encountered. Two general types of passive mixers have been employed, a single balanced mixer and a double balanced mixer. Each type of passive mixer suffers from an inability to switch at appropriate times when the incoming signal includes a relatively large carrier signal and a relatively small modulation signal.
A receiver with good performance that reduces or eliminates nonlinearities in a passive mixer in the presence of a large carrier signal and a relatively small modulation signal is thus desirable.